1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particular to a circular-polarization-based vertical-alignment-mode liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device has various features such as thickness in size, light weight, and low power consumption. The liquid crystal display device is applied to various uses, e.g. OA equipment, information terminals, timepieces, and TVs. In particular, a liquid crystal display device comprising thin-film transistors (TFTs) has high responsivity and, therefore, it is used as a monitor of a mobile TV, a computer, etc., which displays a great deal of information.
In recent years, with an increase in quantity of information, there has been a strong demand for higher image definition and higher display speed. Of these, the higher image definition is realized, for example, by making finer the array structure of the TFTs.
On the other hand, in order to increase the display speed, consideration has been given to, in place of conventional display modes, an OCB (Optically Compensated Birefringence) mode, a VAN (Vertically Aligned Nematic) mode, a HAN (Hybrid Aligned Nematic) mode and a π alignment mode, which use nematic liquid crystals, and an SSFLC (Surface-Stabilized Ferroelectric Liquid Crystal) mode and an AFLC (Anti-Ferroelectric Liquid Crystal) mode, which use smectic liquid crystals.
Of these display modes, the VAN mode, in particular, has a higher response speed than in the conventional TN (Twisted Nematic) mode. An additional feature of the VAN mode is that a rubbing process, which may lead to a defect such as an electrostatic breakage, can be made needless by vertical alignment. Particular attention is drawn to a multi-domain VAN mode (hereinafter referred to as “MVA mode”) in which a viewing angle can be increased relatively easily.
In the MVA mode, for example, mask rubbing and pixel electrode structures are devised, or a protrusion is provided within a pixel. Thereby, the inclination of an electric field, which is applied to the pixel region from the pixel electrode and counter-electrode, is controlled. The pixel region of the liquid crystal layer is divided into, e.g. four domains such that the orientation directions of liquid crystal molecules are inclined at 90° to each other in a voltage-on state. This realizes improvement in symmetry of viewing angle characteristics and suppression of an inversion phenomenon.
In addition, a negative phase plate is used to compensate the viewing angle dependency of the phase difference of the liquid crystal layer in the state in which the liquid crystal molecules are oriented substantially vertical to the major surface of the substrate, that is, in the state of black display. Thereby, the contrast (CR) that depends on the viewing angle is improved. Besides, more excellent viewing angle/contrast characteristics can be realized in the case where the negative phase plate is a biaxial phase plate having such an in-plane phase difference as to compensate the viewing angle dependency of the polarizer plate, too.
In the conventional MVA mode, however, since each pixel has a multi-domain structure, a region, where liquid crystals are oriented in a direction other than a desirable direction, is formed. For example, liquid crystals are schlieren-oriented or orientated in an unintentional direction, at a boundary of the divided domains, at a protrusion in the multi-domain pixel, or near a pixel electrode slit.
The transmittance Tlp(LC) of a liquid crystal layer, under crossed Nicols, of a liquid crystal display device, which uses a linear polarizer plate and executes a linear-polarization-based birefringence control, is expressed by
                              T          ⁢                                          ⁢          1          ⁢                      p            ⁡                          (              LC              )                                      =                              I            0                    ·                                    sin              2                        ⁡                          (                              2                ⁢                θ                            )                                ·                                    sin              2                        ⁡                          (                                                                    Δ                    ⁢                                                                                  ⁢                                                                  n                        ⁡                                                  (                                                      λ                            ,                            V                                                    )                                                                    ·                      d                                                        λ                                ⁢                π                            )                                                          (        1        )            
In equation (1), I0 is the transmittance of linearly polarized light that is parallel to the transmission axis of the polarizer plate, θ is the angle between the slow axis of the liquid crystal layer and the optical axis of the polarizer plate, V is a voltage applied, d is the thickness of the liquid crystal layer, and λ is the wavelength of incident light to the liquid crystal display device.
In equation (1), the refractive index anisotropy Δn(λ, V) depends on an effective application voltage in the region and the inclination angle of each nematic liquid crystal molecule. In order to vary T(LC) to 0 to I0, it is necessary to vary Δn(λ, V)d/λ in a range of 0 to λ/2 and to set the value of θ at π/4(rad). Consequently, in the region where the liquid crystal molecules are oriented in a direction other than π/4, the transmittance decreases. As mentioned above, in the MVA mode, the multi-domain structure is adopted and thus such a region is necessarily formed. Hence, in the MVA mode, a problem, such as low transmittance, occurs, compared to the TN mode.
In order to overcome this problem, a circular-polarization-based MVA mode has currently been studied. The above problem is solved by replacing the linear polarizer plate with a circular polarizer plate, which has a phase plate, that is, a uniaxial ¼ wavelength plate that provides a phase difference of a ¼ wavelength between light rays of predetermined wavelengths that travel along the fast axis and slow axis. The transmittance Tcp(LC) of a liquid crystal layer, under crossed Nicols, of a liquid crystal display device, which uses a circular polarizer plate and executes a circular-polarization-based birefringence control, is expressed by
                              Tcp          ⁡                      (            LC            )                          =                              I            0                    ·                                    sin              2                        ⁡                          (                                                                    Δ                    ⁢                                                                                  ⁢                                                                  n                        ⁡                                                  (                                                      λ                            ,                            V                                                    )                                                                    ·                      d                                                        λ                                ⁢                π                            )                                                          (        2        )            
As is understood from equation (2), the transmittance Tcp(LC) does not depend on the orientation direction of liquid crystal molecules. Thus, a desired transmittance can be obtained only if the inclination of liquid crystal molecules can be controlled, despite the formation of a region where liquid crystals are oriented in a direction other than a desirable direction, for example, a region where liquid crystals are schlieren-oriented or orientated in an unintentional direction at a boundary of the divided domains and near the multi-domain structure.
In the prior-art circular-polarization-based MVA mode, however, there is such a problem that the viewing angle characteristic range is narrow.
FIG. 7 shows an example of the cross-sectional structure of a prior-art liquid crystal display device of a circular-polarization-based MVA mode. As is shown in FIG. 7, a first substrate 13 has a common electrode 9 of ITO (indium tin oxide) on an inner surface thereof. The common electrode 9 is provided with a protrusion 12 for forming a multi-domain structure within a pixel. A second substrate 14, which is opposed to the first substrate 13, has a pixel electrode 10 of ITO on an inner surface thereof. The second substrate 14 has slits 11 (where no pixel electrode is provided) for forming the multi-domain structure within the pixel. A nematic liquid crystal 7 with negative dielectric anisotropy is sandwiched between the common electrode 9 and pixel electrode 10. An orientation process is executed such that liquid crystal molecules 8 are aligned substantially vertical to the major surface of the substrate in a voltage-off state.
The liquid crystal cell with the above structure includes phase plates 3 and 4 and polarizer plates 5 and 6, which are provided on both outer surfaces of the liquid crystal cell. The phase plate 3, 4 is a uniaxial ¼ wavelength plate having refractive index anisotropy (nx>ny=nz). The slow axis of the phase plate 3, 4 has an angle of π/4 (rad), relative to the transmission axis of the polarizer plate 5, 6.
In the above structure, the paired phase plates 3 and 4 are configured to have slow axes that are intersect at right angles with each other, and thus function as negative phase plates. For example, a negative phase difference of about −280 mm is imparted to light with a wavelength of 550 nm. On the other hand, in order to obtain a phase difference of ½ wavelength by an electric field control, the liquid crystal layer 7 needs to have the value of Δn·d of 300 nm or more, which is obtained by multiplying the refractive index anisotropy Δn of the material by the thickness d of the liquid crystal layer. Consequently, the total phase difference of the liquid crystal display device does not become zero, and the viewing angle characteristics at the black display time deteriorate. In addition, since the uniaxial ¼ wavelength plate is used, a viewing angle dependency occurs in polarization characteristics of circularly polarized light that enters the liquid crystal layer, owing to the viewing angle characteristics of the polarizer plate.
As described above, in the prior-art circular-polarization-based MVA mode, substantially circularly polarized light is produced as the incident light that enters the liquid crystal layer. Thereby, the above-mentioned problem of low transmittance is overcome. However, there is such a problem that the contrast/viewing angle characteristic range is narrow because of lack of means for compensating the viewing angle dependency of circularly polarized light, which enters the liquid crystal layer, or the viewing angle dependency of the phase difference of the liquid crystal layer.
FIG. 8 shows an example of the measurement result of isocontrast curves of the liquid crystal display device having the structure shown in FIG. 7. The 0 deg. azimuth and 180 deg. azimuth correspond to the horizontal direction of the screen, and the 90 deg. azimuth and 270 deg. azimuth correspond to the vertical direction of the screen. As is shown in FIG. 8, the viewing angle with a contrast ratio of 10:1 is ±40° in the vertical direction and horizontal direction, and is narrow. Practically tolerable characteristics are not obtained.
An approach to address this problem has been proposed, wherein the uniaxial ¼ wavelength plate is replaced with a biaxial ¼ wavelength plate having refractive index anisotropy (nx>ny>nz) as shown in FIG. 10, thereby compensating the viewing angle dependency of circularly polarized light that enters the liquid crystal layer, and improving the viewing angle characteristics.
FIG. 9 shows an example of the cross-sectional structure of the circular-polarization-based MVA mode liquid crystal display device that uses biaxial ¼ wavelength plate 15 as shown in FIG. 10. In this structure, the ¼ wavelength plate has a refractive index ellipsoid of nx>ny>nz, as shown in FIG. 10. Thus, the in-plane phase difference is ¼ wavelength. If the upper and lower ¼ wavelength plates are disposed such that their in-plane slow axes intersect at right angles with each other, they function as negative phase plates. If their phase difference value is controlled, the phase difference in the normal direction of the liquid crystal layer can be compensated, and the viewing angle characteristics are improved.
FIG. 11 shows an actual measurement result of isocontrast curves of the circular-polarization-based MVA mode liquid crystal display device shown in FIG. 9. Compared to the result shown in FIG. 8, it is understood that the viewing angle is slightly increased and the characteristics are improved. However, the viewing angle with a contrast ratio of 10:1 or more is about ±80° and is wide in the oblique directions, but the viewing angle with a contrast ratio of 10:1 or more is about ±40° in the vertical and horizontal directions, which fails to satisfy practically tolerable viewing angle characteristics. The reason is as follows. The phase difference in the normal direction of the liquid crystal layer is improved to some degree by the above-described biaxial ¼ wavelength plates. An actually usable film, however, is a high-polymer film, and it is difficult to match it with wavelength dispersion of the phase difference of the liquid crystal layer. Furthermore, the film, as a circular polarizer plate, does not have such a structure as to have sufficient viewing angle characteristics, and this leads to the above-mentioned viewing angle characteristics of the contrast ratio.
To solve the problem, a circular-polarization-based MVA mode liquid crystal display device has been proposed, which uses a biaxial ¼ wavelength plate having a refractive index anisotropy as shown in FIG. 13, in place of the biaxial ¼ wavelength plate shown in FIG. 10.
FIG. 12 shows an example of the cross-sectional structure of a circular-polarization-based MVA mode liquid crystal display device that uses the biaxial ¼ wavelength plate 16 shown in FIG. 13. In this structure, the ¼ wavelength plate has a refractive index ellipticity of nx>ny<nz, as shown in FIG. 13. Like the structures shown in FIG. 7 and FIG. 9, the ¼ wavelength plates 16 and polarizer plates 5 and 6 are disposed on the outer surfaces of the MVA mode liquid crystal cell.
In the structure shown in FIG. 12, the ¼ wavelength plate that is used has a refractive index of ny<nz. Thus, even in the case where nx>nz and the ¼ wavelength plates are disposed above and below the liquid crystal cell so as to have slow axes perpendicular to each other, the effect of the negative phase difference is weakened, compared to the structure shown in FIG. 7 in which the upper and lower uniaxial ¼ wavelength plates are disposed to be perpendicular to each other. In the case where nx<nz, a positive phase difference occurs. Consequently, the contrast/viewing angle characteristic range becomes narrower than in the structure shown in FIG. 7, unless the refractive index anisotropy Δn of the liquid crystal layer is set to be very small, that is, unless the variation in phase difference of the liquid crystal layer is set below ½ wavelength and the transmittance of the liquid crystal cell becomes insufficient.
FIG. 14 shows an actual measurement result of isocontrast curves of the circular-polarization-based MVA mode liquid crystal display device shown in FIG. 12. As shown in FIG. 14, there occurs a region where the contrast ratio is 1:1 or less, and it is understood that the viewing angle characteristic range is narrower than in FIG. 8 or FIG. 11. This is partly because the structure of the polarizer plate, like the structure shown in FIG. 9, is not configured to obtain sufficient viewing angle characteristics as a circular polarizer plate.
Each of the structures shown in FIG. 9 and FIG. 12 uses the biaxial ¼ wavelength plate. The biaxial phase plate is formed by biaxial-drawing a high-polymer film, which leads to an increase in manufacturing cost. In addition, the refractive index is controllable only in a limited range, and it is difficult to realize a desired refractive index ellipsoid. Moreover, the range of selection of material for obtaining biaxiality is narrow, and it is difficult to match the material with the wavelength dispersion characteristic of the refractive index of the liquid crystal (see, for instance, T. Ishinabe et al., A Wide Viewing Angle Polarizer and a Quarter-wave Plate with a Wide Wavelength Range for Extremely High Quality LCDs, IDW '01 Proceedings, p. 485 (2001), and Y. Iwamoto et al., Improvement of Display Performance of High Transmittance Photo-Aligned Multi-domain Vertical Alignment LCDs Using Circular Polarizers, IDW '02 Proceedings, p. 85 (2002)).